Tim Radford reports on problems in bringing Mir down to Earth
The troubled Mir spacecraft could land on populated areas when it ends its 13-year endurance trial in orbit, say British scientists.
Russian scientists plan to nudge the elderly space hotel and laboratory into the atmosphere and lose it in the Pacific Ocean at the end of next year, but British experts are warning that it could hit populated areas instead.
Dr Richard Crowther, a space scientist at Dera, Britain’s defence research agency, says: “The trouble is that Mir is a very complex vehicle. It’s very easy to de-orbit a vehicle such as the shuttle, which has well-defined and symmetric aerodynamic surfaces.
“The problem with the Mir station is that it is asymmetric and it is difficult to predict how such a shape would behave when it gets into the lower reaches of Earth’s atmosphere. Unlike the shuttle, which has rigid surfaces, the Mir station has solar arrays which will bend and buckle quickly.”
The last Nasa astronaut on Mir – the Australian-born Andrew Thomas – has checked out, leaving only a Russian crew. Moscow space chiefs may decide they cannot afford to supply the space station for much longer. Its managers plan to let it sink gradually closer to Earth, and then in December 1999 help the last supply ship nudge it into the atmosphere on a trajectory that will let it splash down harmlessly.
Crowther says that once Mir has been pushed into the atmosphere it would be difficult to control its terminal trajectory. Mir would come streaking from the heavens at a shallow angle at 28 000kph, heated, braked and buffeted by an increasingly thick atmosphere.
“If you are out by several minutes, you could be out by hundreds of kilometres. The issue is where the station would end up,” he says.
Mir’s forerunner, the Salyut-7 spacebase, came back in 1991. It should have landed in the Atlantic. “It ended up striking South America. The Americans had a similar experience with Skylab. Again they aimed for the Pacific and ended up going into Western Australia. It seems that, even though people have very large targets, just because of the complex configurations of these vehicles it’s difficult to predict where they are going to fall along the track.”
Mir’s orbital pathway could take it as far north as London, as far south as the Falkland Islands. But the worry is where along the ground track the larger pieces will fall. Mir is a large object: 90% of it is likely to vaporise in the heat of re-entry into the atmosphere. But it weighs 140 tons. That could still leave 14 tons of metal heading for places of habitation at several hundred kilometres an hour.
There is also a joker in the pack. December 1999 is expected to be a “solar maximum” – the high point in the sun’s 11-year cycle. There should be more radiation from the sun, which would in turn heat Earth’s atmosphere and make it expand. This has already proved a hazard for satellites in low orbit, causing them to slow unexpectedly and fall.
“It’s like a sun-tanned grim reaper that appears every 11 years,” says Crowther. “It comes out and grabs the satellites and brings them back to Earth.”
Space agencies routinely plan early in the design stage how spacecraft will end their lives, but the Mir programme is 13 years old. It has already survived far longer than its planned life. In the last couple of years there has been a series of sudden, terrifying moments, including a collision with a supply vessel and a fire.
Mir’s crew has also had to deal with regular computer failure, power loss, spilled chemicals, uncertain oxygen supply and overflowing lavatories. Even so, Mir has had a key role in providing endurance training in low gravity for US astronauts who will be working on the planned 40- billion international space station next year.
“It’s actually achieved a great deal since it has been up there,” says Crowther.